On site B site-to-site settings add 192.168.2.0/24 to the "Remote Networks" to get the traffic routed over the vpn.
On site A in access server settings add 192.168.126.0/24 to the "Local Networks" to get the route for this network pushed to the clients.

For the site-to-site vpn you should have assigned interfaces at both sites.
Ensure that your firewall rules allow the access between the networks.

I was trying what you suggested but I have a problem. Applying first two configurations all continue working well, but when I try to assign interfaces in site A, adding firewall rules, the comunication between site A and site B stops, also if the VPN is up.
Can you suggest me more specifically what I have to do? Can you tell me in which interface I have to configure firewall rules?

Just assign an interface to the openvpn stite to site server at A, open the interface settings and enable it. You also may set a name if you like, but do no further configurations, no IP settings!
Also assign an interface to the openvpn client at B this way.

Consider that firewalls rule on the interfaces control incoming traffic. So if you want to allow access from site B LAN to A LAN you need a firewall rule on the LAN interface at B which allow the traffic and also you need a rule on the newly added vpn interface at A allowing the traffic.